How to mark the seams of UV maps to edit textures in external programs?
If you are painting some textures that are a little bit more complex, you might want to consider doing that on the 3d model in 3d. There are plenty of tools for that in a lot of 3d packages these days, Blender included. Obviously if the textures you need are simple, using an external image editor is a very valid option as well. Especially for every day simple objects. 

The usual workflow is to export the UV map as an image and make the textures using that as a guide. 

You would simply enter edit mode, select all to view the UV map, go to the UV Editor and choose UVs menu on the header, then Export UV Layout:



You can chose desired options and save the file then.

It is then usual to open it in an external editor and have it as a top layer while working on the texture so you can see the location of the UV islands while you work and disable it once you save your texture: